The present invention relates to developer-finisher compositions for photographic elements comprising photosensitive coatings on substrates, or more particularly to developer-finisher compositions for removing and desensitizing the non-image areas of exposed, usually negative working lithographic printing plates.
Lithographic printing plates generally are composed of an aluminum containing substrate which may or may not have been treated with a variety processes recognized in the art process including anodization, graining and hydrophilization. The thusly prepared substrate may then be applied with a photosensitive coating comprising a photosensitizer, binding resins, colorants, acid stabilizers, surfactants and other art recognized components. Common photosensitizers include diazo compounds, including polymeric diazonium condensates salts and photopolymerizable compositions. Sensitizers, binders and printing plates employing aromatic diazonium compounds are described in U.S. Pat. Nos. 3,175,906; 3,046,118; 2,063,631; 2,667,415; 3,867,147 and 3,679,419 which are incorporated herein by reference.
It is known in the art to prepare developing compositions for lithographic plates. Likewise desensitizing finishers are also known. Attempts have been made in the past to prepare single composition developer/finishers, however, these suffer from drawbacks such as incompatibility of the components. In general, one cannot simply mix a developer and a finisher to obtain a useful developer/finisher. Developer compositions remove the non-image portion of the plate after it has been imagewise exposed. A finisher must function to desensitize the non-image areas to assure that they will not accept greasy ink upon printing. The finisher also prevents oxidation of the background areas of the plate during storage or while waiting for press mounting. The finisher must also be quickly removable from the plate so that it will not cause production delays. Typically the finisher must be quickly removable by a water rinse or most preferably must be removable by the fountain solution used on the press. Quick rollup is then essential in order to prevent paper waste and reduced production time. The developer/finisher should also bind removed non-image particles of the photosensitive surface so they are not re-deposited back onto the plate.
In the past, various methods of developing printing plates have been employed. Such methods include the use of compositions containing organic solvents, surfactants, salts, acids and other art recognized materials in solution. Typically prior art developing compositions are described in U.S. Pat. No. 2,754,279; 4,381,340 and German OLS 2,216,419. It is most desirable to have a developer composition which is capable of completing development in a short period of time. The oleophilic image areas of the printing plate should be ink receptive and water repellant whereas the hydrophilic non-image areas should be water receptive and greasy ink repellant.
The proper selection of the solvent in a developer solution is very important. If the selected solvent in a developer solution is a good solvent of the light sensitive composition itself, the solvent will not satisfactorily distinguish between the exposed areas and the unexposed areas. As a result, a large portion of the image area coating is dissolved in the solvent or is severely swollen and removed. Such printing plates have no practical value. By the addition of a large amount of water to reduce the concentration of the organic solvent in the developer solution, image differentiation can be improved. However, during the development of a printing plate, particularly in negative working plates, nonimage areas comprising resin binders such as acetals, and particularly polyvinyl formal, solvate in the developer solution and separate from the support and contract to spherical shapes in a highly tacky condition. Such solvated particles may otherwise deposit upon still-solvent-wet image areas and adhere permanently even after drying. Not only are these "redeposited" areas visible to the naked eye on the developed printing plate but are removed by tacky ink during printing which pulls away the true underimage, leaving skips which show as absence of image on the printed sheet. A balance of developer properties is necessary where adequate speed of development is attained with a minimum of redeposited particles.
Efforts have been made to prepare developer/finishers which, as the name suggests, develop and prepare a plate for the printing press or for storage in a single operation. This objective has become particularly desirable with the advent of processing machines. Examples of such solutions include U.S. Pat. No. 2,754,279 and German Offenlegungsschrift No. 2,216,419. The latter two patents describe two-phase developer/finishers.
In the use of both developers and developer/finishers in machines, problems arise in the etching and plating of the process as a result of the ingredients of the developer or developer/finisher.
It is an object of this invention to provide a developer and developer/finisher for both machine and hand development of lithographic printing plates having resin binders in their photosensitive coatings, which will enable the operator to prepare a plate free from objectionable redeposit.
It is another object of this invention to provide processing solutions which combine the above objects with good hydrophilization of the nonimage areas and oleophilization of the image areas.
These and other objects will become apparent in the detailed description of the preferred embodiment.